When light hits chlorophyll a few things can happen

When pigments absorb photons an election gets excited

Light energy to chemical energy through photosystems (II & I)

Light reactions: make ATP + NADPH for Calvin cycle


Calvin cycle (in stroma) uses products from light reaction

Calvin cycle is a 3-step process to produce sugars


  • 1. Fixation: CO2 reacts with 5C molecule (RUBP)
    • new 6C split into 2 acids (3C each)
    • catalyzed by Rubisco enzyme


  • 2. Reduction: ATP and NADPH turn 3C acids into sugars
    • reduction of acids
    • from donation of electrons


  • 3. Regeneration: Some 3C sugars recycled to make RUBP (5C)
    • one 3C sugar leaves to make glucose
    • recycling also needs ATP

Making sugars requires several cycles (for regeneration)

Rubisco - the enzyme that dominates carbon fixation






  • Most abundant enzyme in the world


  • Most CO2 converted into biomass is fixed by Rubisco


  • Large molecule : 16 polypeptides : 8 active sites

Rubisco: the clunky and slow carbon fixer




  • Only 3-10 reactions per second
    • limits rate of photosynthesis


  • 20% error rate
    • gets worse at high temperatures


  • Also reacts with O2 (cheater!)
    • O2 reaction called Photorespiration
    • uses ATP and NADPH to make CO2

Photosynthesis is not perfect


  • Evolution doesn’t always create the best solution


  • Photorespiration is completely wasteful
    • 2x energy to produce the same amount of sugar than if Rubisco only reacted with CO2


  • Rubisco has evolved high affinity for CO2 (80x > O2)
    • compare to the composition of the atmosphere…


  • C3 photosynthesis is dominant, but other types of evolved

Rubisco and plant evolution: How did we get here?





  • Why is there so much Rubsico?


  • Why is it so big and slow?


  • Was Rubisco always a cheater?


  • Why did the atmosphere change?

Why has Rubisco not been replaced?



  • Evolution of enzymes difficult
    • favors stability
    • selection against modifying mutations


  • RubisCO → RubisC???
    • requires a decrease in fitness
    • plants are stuck with Rubisco


  • Natural selection increased affinity for CO2
    • made Rubisco slow

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